Glow plugs of the type usually used employ a thin metal sleeve which is set into a socket. The metal sleeve is closed at one end. An electrically insulating material which is a good conductor of heat is included in the closed sleeve. An electrical heating resistance wire is coiled within the insulating material. The assembly of the metal sleeve with the insulating material and the heating wire is fixed in a socket where it must be securely held.
It is difficult to connect the metal sleeve to the socket. If welding is used, difficulties arise due to the substantial difference in wall thickness between the sleeve and the socket. The wall thickness of the sleeve should be as thin as possible to provide good heat transfer from the resistance wire and to ensure rapid heating.
In some plugs, the heating or glow sleeve has been formed like a pin and, by using special sealing materials, the sleeve has been screwed into the socket. In another construction, a metal sleeve is held in the socket by means of a glass melt. These methods of securing the sleeve to the socket are expensive to carry out and do not satisfy long operating requirements particularly with respect to the seal between the metal sleeve and the socket. If has also been proposed to secure the sleeves in the sockets by means of hard-soldering or brazing. This step can be carried out only in an atmosphere of protective gas in order to obtain a reliable solder or brazing connection. The solder connection results in a good plug which satisfies functional requirements; the method of manufacturing such a plug is, however, expensive. A further problem arises in soldering or brazing: sealing elements which may be placed at the terminal end of the glow plug may be damaged when introducing sufficient heat to effect brazing or welding, if the terminal end is at, or close to the soldering or brazing region. To protect any non-metallic elements it is necessary to extend the length of the glow plug so that the end portion is sufficiently for removed from the solder or brazing region.
A widely used method to connect the glow plug sleeve and the metal socket is: The glow tube is introduced into a bore of the socket with some play; the portion of the socket which is closest to the combustion chamber -- when the glow plug is installed in an internal combustion engine, such as a Diesel engine -- is then compressed from the outside. This requires quite complicated machinery which is expensive.
It has also been proposed to connect the thin-walled glow plug metal sleeve with the socket by shrinking a sleeve of steel on the thin-walled glow plug sleeve and then press-fitting the steel bushing into a bore of the socket. Rather than shrinking the steel bushing on the glow plug tube, it has also been proposed to solder the sleeve thereon. This solution to the manufacture of glow plugs has not found favor in industry due to the high costs and the difficulty of effecting a reliable connection.